Dielectric elastomers (DE's) have been studied as a candidate active material a for morphing aircraft, synthetic jets artificial muscle and numerous other active material applications. Prior dielectric elastomer devices exhibited large actuation strains, but low output forces. The low shear modulus and equally small tensile stiffness of the material are responsible for the low force output.
Prior DEs, such as DE 10 illustrated in FIG. 1, begin with a sheet form of polyacrylate or other fully-cured elastomer 12. The most common elastomer used in current DE devices is polyacrylate adhesive tape, which is commercially available in 0.50 mm and 1.00 mm thicknesses. This elastomer is a permanently tacky compound used in high performance double-sided adhesive applications. The use of an existing fully-cured sheet form limits the options for fiber arrangement. In addition, the sheet form makes the introduction of compliant electrodes difficult. Other embodiments use elastomer tubing for cylindrical actuators. These are also limited by the available thicknesses of cured elastomer tubing forms.
The cured polyacrylate elastomer 12 is sandwiched between two compliant electrodes 14. FIG. 1A shows the prior DE without voltage applied, and FIG. 1B shows the prior DE with voltage applied. When voltage is applied, the DE decreases in thickness (illustrated by the vertical arrows in FIG. 1B) and increases in size in the in-plane direction (illustrated by the horizontal arrows in FIG. 1B).
Prior DEs employ a highly compliant electrode to carry the necessary electrical charge while allowing for the large displacements in the in-plane direction. This has been accomplished by a series of conductive fluid carriers (greases or pastes) with various conductive materials (carbon, silver, carbon nanotubes, etc.) in sufficient quantities to percolate, or form a conductive network capable of transporting the electrical charge during actuation. Use of fluid-like electrodes makes assembly difficult and limits the reliability and resistance to environmental hazards such as dust dirt, chemicals, and UV exposure. The grease and paste electrodes create similar problems for planar as well as cylindrical DE devices.
Another problem plaguing prior DE systems is the need for very large voltages (>2000 volts) to actuate the material. The voltage required is inversely proportional to the distance between electrodes. The fixed-thickness sheet form fixes the electrode spacing, making it impossible to reduce the operating voltages.
Another issue with prior DEs is the stress state under which they operate. Elastomers are weak in shear and weak in tension, but have a very high bulk modulus. Prior DE devices load the material in tension and rely on their near incompressibility to convert actuation in one direction to displacement in an orthogonal plane. Unfortunately, this loads the material in tension and accounts for the low displacement forces. Prior DE actuators expand upon activation. For this reason, devices employing them must be designed to provide prestrain prior to the application of electrical charge.